Hereinafter, a “Q” prefix in a word or phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (Q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits”) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These is and Os act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor using superconducting qubits (IBM is a registered trademark of International Business Machines Corporation in the United States and in other countries.)
A superconducting qubit includes a Josephson junction. A Josephson junction is formed by separating two thin-film superconducting metal layers by a non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature-pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a qubit, the Josephson junction—which functions as a dispersive nonlinear inductor—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear microwave oscillator. The oscillator has a resonance/transition frequency determined by the value of the inductance and the capacitance in the qubit. Any reference to the term “qubit” is a reference to a superconducting qubit oscillator circuitry that employs a Josephson junction unless expressly distinguished where used.
The information processed by qubits is carried or transmitted in the form of microwave signals/photons in the range of microwave frequencies. The microwave frequency of a qubit output is determined by the resonance frequency of the qubit. The microwave signals are captured, processed, and analyzed to decipher the quantum information encoded therein. A readout circuit is a circuit coupled with the qubit to capture, read, and measure the quantum state of the qubit. An output of the readout circuit is information usable by a Q-processor to perform computations.
A superconducting qubit has two quantum states—|0> and |1>. These two states may be two energy states of atoms, for example, the ground (|0>) and first excited state (|1>) of a superconducting artificial atom (superconducting qubit). Other examples include spin-up and spin-down of the nuclear or electronic spins, two positions of a crystalline defect, and two states of a quantum dot. Since the system is of a quantum nature, any combination of the two states is allowed and valid.
For quantum computing using qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other parts of the quantum processor, must not alter the energy states of the qubit, such as by injecting or dissipating energy, in any significant manner or influence the relative phase between the |0> and |1> states of the qubit. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and superconducting structures that are used in such circuits.
The illustrative embodiments recognize that a qubit's resonance frequency is inherently fixed at the time the qubit is fabricated, i.e., when the Josephson Junction and the capacitive element of the qubit-oscillator are fabricated on a Q-processor chip. The illustrative embodiments further recognize that in the simplest implementation of a quantum processor (Q-processor), at least two qubits are needed to implement a quantum logic gate (Q-gate). Therefore, a Q-processor chip is typically fabricated to have at least 2, but often 8, 16, or more qubits on a single Q-processor chip.
Some qubits are fixed-frequency qubits, i.e., their resonance frequencies are not changeable. Other qubits are frequency-tunable qubits. A Q-processor can employ fixed-frequency qubits, frequency-tunable qubits, or a combination thereof.
The illustrative embodiments recognize that it is difficult to fabricate single-junction transmons or fixed-frequency superconducting qubits with specific accurate frequencies or accurate frequency differences between neighboring qubits. This is mainly because the critical current of Josephson junctions is not a well-controlled parameter in the fabrication process. This results in a relatively wide-spread in the critical currents of Josephson junctions having the same design and area and fabricated on the same chip.
The illustrative embodiments recognize that when the resonance frequencies of two neighboring coupled qubits on a chip are the same or within a threshold band of frequencies or their higher transition frequencies are on resonance or close to resonance, then negative effects can happen such as, crosstalk, quantum decoherence, energy decay, creation of mixed states, unintended information transfer, quantum state leakage and so on. Having such qubits can also negatively affect the performance or utility of certain quantum gates such as cross-resonance gates which have stringent requirements on the spectrum of resonance frequencies of qubits upon which the gate is operating on. Therefore, the illustrative embodiments recognize that one challenge in quantum processors that are based on coupled fixed-frequency qubits is frequency crowding or frequency collision between adjacent qubits, in particular, when cross-resonance gates are used.
It is important to note that while the proposed flux-biasing technique is motivated by the need to solve frequency collisions of coupled qubits on the same chip which are acted on with cross-resonance gates, the proposed flux-biasing technique is general, and can be applied to other kinds of quantum devices on chip which require relatively high-density flux biasing without penetrating the device package.
The illustrative embodiments recognize that a frequency-tunable qubit (hereinafter compactly referred to as a “tunable qubit”) has a flux-dependent inductance, consisting of a superconducting loop that includes one or more Josephson junctions. By varying the magnetic field threading the loop, the inductance of the loop changes, which in turn changes the resonance frequency of the qubit, thus making the qubit tunable. The illustrative embodiments recognize that one challenge in quantum processors that are based on tunable-frequency qubits is sensitivity to flux noise which leads to dephasing.
Presently, when the frequency of a flux-tunable qubit on a chip has to be changed, there are two main methods that are used in the state-of-the-art to apply or change the flux threading the loop of the qubit. The first method is using a global superconducting coil attached to the qubit-chip package. This method has the advantage of having an external fully controllable magnetic source which does not penetrate the device package. Such an external source can be filtered well and avoids several negative affects of having magnetic field lines inside of the package and near the quantum chip such as, crosstalk, power leakage, noise penetration. The disadvantage of this method is that the qubits cannot be individually controlled and tuned. The second method is using on-chip magnetic field lines or flux-lines that are placed on a different layer or printed circuit board that are near the qubit chip. The advantages of this method are: 1—it is scalable, 2—enables high-density flux-line systems for large quantum processors, 3—allows individual qubits to be tuned and controlled. The disadvantages of this method are: 1—it introduces many possible noise channels between the quantum processor and the external environment which can negatively affect the coherence and performance of the quantum processors, 2—it is difficult to fabricate and route the on-chip flux-lines or magnetic field lines that are near the qubits on different layers or on printed circuit boards inside the device package.